1. Field of the Invention
The present invention is broadly concerned with new polyoxometalate compounds and polymers comprising recurring monomers of those compounds. More particularly, the inventive compounds comprise an organoimido group bonded to the polyoxometalate in place of at least one oxide of each starting polyoxometalate compound so as to improve certain properties (e.g., solubility in organic solvents) of the compounds.
2. Description of the Prior Art
Polyoxometalates are soluble, inorganic cluster-like compounds formed principally of an oxide anion and early transition metal cations. These discrete polymeric structures form spontaneously when simple oxides of, for example, vanadium, niobium, tantalum, molybdenum, or tungsten are combined under appropriate conditions in water. In a great majority of polyoxometalates, the transition metals are in the d0 electronic configuration which dictates both high resistance to oxidative degradation and an ability to oxidize other materials. The principal transition metal ions that form polyoxometalates are tungsten(VI), molybdenum(VI), vanadium(V), niobium(V), and tantalum(V).
Isopolyoxometalates, the simplest of the polyoxometalates, are binary oxides of the formula [MmOy]pxe2x88x92, where m may vary over a wide range of numbers. For example, if m=8 and M=Mo, then the formula is [Mo8O26]4xe2x88x92.
Heteropolyoxometalates have the general formula [XxMmOy]pxe2x88x92 and possess a heteroatom, X, at the center thereof. For example, in the xcex1-Keggin structure, xcex1-[PW12O40]3xe2x88x92, X is a phosphorus atom. The central phosphorus atom is surrounded by twelve WO6 octahedra.
Polyoxometalates are characterized by a number of useful structural, electrochemical, catalytic, magnetic, medicinal, and photophysical properties (see, e.g., Chemical Reviews, 98:1-389 (1998), incorporated by reference herein). Examples of applications of polyoxometalate systems include: solid state electrochromic devices; electrochemical fuel cells, precursors of oxide films for optoelectronics, recording materials, electrophotography, corrosion-resistant coatings, capacitors, and flammability control/smoke suppression. In the vast majority of these applications, the polyoxometalate species is utilized as an additive, a co-precipitant, or an ionic dopant. That is, the polyoxometalate species is present as a heterogenous additive rather than as a covalently-bonded integral component within the device. There are several disadvantages to using the polyoxometalate species as a heterogenous additive, including a non-uniform distribution of the polyoxometalate species, difficulties in varying the amount of polyoxometalate incorporation, migration and/or loss of polyoxometalate, and poor processability.
There are very few previous examples of polymers bearing covalently-incorporated polyoxometalate species. Knoth, J. Am. Chem. Soc., 101:2211 (1979), incorporated by reference herein, has described an all-inorganic polymer [(OC)3CoGe2W11SiO405xe2x88x92]n. Katsoulis et al., Mater. Res. Soc. Symp. Proc., 435:589 (1996), incorporated by reference herein, have described siloxane polymers bearing polyoxometalate species. Finally, Judeinstein, Chem. Mat., 4:4-7 (1992), incorporated by reference herein, has described the homo-polymerization of systems such as [Bu4N]4[SiW11O39(RSiOSiR)], wherein R is vinyl, styryl, allyl, or methacryl group.
The present invention is broadly concerned with new polyoxometalate compounds as well as polymers comprising recurring monomers of these compounds. These compounds are formed by covalently bonding an organoimido group (NR, where R comprises a polymerizable moiety) to a metal site of the polyoxometalate compound in place of an oxide group.
In one embodiment polyoxometalate compounds according to the invention are represented by the formula:
[Qx]+a[XbMmMxe2x80x2pMxe2x80x3zOn(NR)yYc]xe2x88x92a
wherein:
each of M, Mxe2x80x2, and Mxe2x80x3 is a metal individually selected from the group consisting of Mo, W, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Ru, Rh, Ta, Re, and Os;
R may be the same or different in each (NR) moiety and each R is individually selected from the group consisting of substituted and unsubstituted alkyl and aryl groups other than styrene groups;
each X is individually selected from the group consisting of Si, P, B, As, Se, S, Sn, Sb, and Bi;
each Q may be the same or different, with each Q individually being a cation;
each Y is individually selected from the group consisting of H2O and the halides;
b is a number ranging from about 0-10, and preferably from about 1-3;
m is a number ranging from about 1-40, and preferably from about 4-18;
each of p and z is individually a number ranging from 0-6, and preferably from about 1-3, with the sum of m, p, and z ranging from 1-40;
n is a number ranging from about 1-200, preferably from about 3-62, and more preferably from about 6-39;
each a is the same number and ranges from about 1-20, and preferably from about 2-12;
each x is a number ranging from about 1-20, and preferably from about 2-12;
c is a number ranging from 0-2, and preferably from about 1-2; and
y is a number ranging from about 1-20, and preferably from about 2-6.
As used in the above formula, when b, p, z, and/or c is 0, it is intended that the substituent which b, p, z, or c is quantifying is not present in the compound. For example, if b is 0, then X is not present in the particular polyoxometalate compound.
In one embodiment, both p and z are 0, and M is Mo. In another embodiment, at least one of M and Mxe2x80x2 is selected from the group consisting of Mo, W, and V, and Mxe2x80x3 is selected from the group consisting of Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Ru, Rh, Ta, Re, and Os.
In embodiments where a countercation (Q) is present, each countercation is preferably not bonded to the (XbMmMxe2x80x2pMxe2x80x3zOn(NR)yYc) complex, but is instead physically present therewith in order to counteract the negative charge of the complex. The presence of the countercations can be exploited in several ways such as to increase the solubility of the compounds in organic solvents or to provide for desirable properties through interaction with polyoxometalate components. Each Q is preferably individually selected from the group consisting of H+, alkali metal cations, alkaline earth metal cations, substituted and unsubstituted ammonium cations, substituted and unsubstituted phosphonium cations, and metal complex cations, with bis(tetra-n-butylammonium), lithium cations, and mixtures thereof being the most preferred countercations.
In each of the foregoing embodiments, the nitrogen atom of the (NR) moiety is bonded to a metal atom of the (XbMmMxe2x80x2pMxe2x80x3zOnYc) fragment, preferably via a triple bond. Furthermore, R is preferably a C1-C8 alkyl group or a C6-C12 aryl group. Even more preferably, at least one R includes a reactive portion or moiety selected from the group consisting of substituted and unsubstituted vinyl groups (e.g., such as part of an acrylate derivative), substituted and unsubstituted allyl groups, and silyl groups.
When R comprises a vinyl group, that group is represented by the formula xe2x80x94CRxe2x80x2xe2x95x90CH2, wherein Rxe2x80x2 is selected from the group consisting of hydrogen, substituted and unsubstituted alkyls (preferably C1-C6), substituted and unsubstituted aryls (preferably C6-C10), substituted and unsubstituted silanes, substituted and unsubstituted siloxides, substituted and unsubstituted siloxanes. When R comprises an allyl group, that group is represented by the formula xe2x80x94CRxe2x80x32xe2x80x94CRxe2x80x3xe2x95x90CH2, wherein each Rxe2x80x3 is individually selected from the group consisting of hydrogen, substituted and unsubstituted alkyls (preferably C1-C6), substituted and unsubstituted aryls (preferably C6-C10), substituted and unsubstituted silanes, substituted and unsubstituted siloxides, substituted and unsubstituted siloxanes. Finally, when R comprises a silyl group, that group is preferably represented by the formula xe2x80x94SiXxe2x80x2p, wherein each Xxe2x80x2 is individually selected from the group consisting of the halides, alkoxides, alkyls (preferably C1-C8), aryls (preferably C6-C12), alkenyls (preferably C1-C8), and hydrogen, and p is a number ranging from about 1-3.
It will be appreciated that the inventive polyoxometalate compounds can be polymerized (either alone or with other compounds) by conventional polymerization reactions to form new polymers via the R groups on the respective compounds. For example, the desired polyoxometalate compounds according to the invention can be combined in a suitable solvent system with the chosen co-monomer(s) in the desired molar ratio, with a polymerization initiator (if necessary; e.g., 2,2xe2x80x2-azobisisobutyronitrile) being added to the system. Preferably, the compounds are co-polymerized with olefinic monomers and mixtures thereof, and preferably monomers selected from the group consisting of substituted and unsubstituted styrene monomers (e.g., 4-methylstyrene), olefinic monomers (e.g., cross-linking olefins such as divinylbenzenes and silicon-containing olefins such as olefinic silanes, olefinic siloxides, and olefinic siloxanes and their substituted derivatives), acrylates (e.g., methyl methacrylate), vinylic monomers (e.g., vinyl chloride, acrylonitrile, and vinyl acetate), and vinylidene monomers (e.g., vinylidene chloride).
Generally, the solvent systems employed will be non-aqueous systems such as organic solvent systems (e.g., 1,2-dichloroethane and other chlorinated hydrocarbons, toluene, acetonitrile, benzene, and chlorinated aromatic compounds). The polymerization reaction can be carried out over a wide range of temperatures (such as from about 0xc2x0 C. to about the boiling point of the chosen solvent) depending upon the chosen co-monomers and solvent system. The reaction is preferably carried out at ambient pressures in an oxygen-free atmosphere. Once the complex has been polymerized, the original tetrabutylammonium countercations (if used) are no longer required in order to confer solubility, and may be replaced by other desirable cationic entities.
The polymers according to the invention differ from prior art polyoxometalate polymers in that the polyoxometalate complex is linked to the polymer chain by way of a nitrogen atom rather than an oxygen atom. Furthermore, the overall nature of the polyoxometalate moiety itself is different in that at least one of the oxide groups of the corresponding polyoxometalate compound has been replaced by an isoelectronic organoimido group. This modifies the properties of the polyoxometalate in the following ways: the electronic spectral absorptions of the compounds are shifted in energy and are more intense; the polyoxometalates have enhanced solubility in organic solvent systems; and the electrochemical behavior of the polyoxometalates is modified. Advantageously, these modifications can be controlled by varying the nature and identity of the R of the (NR) moiety as well as by varying the number of (NR) moieties incorporated into the polyoxometalate.
The inventive polyoxometalate compounds and polymers are useful in a wide range of applications. For example, because the polyoxometalates comprising Mo can be incorporated into polymers in a uniform and controlled manner, the polymers are useful as flame retardants and smoke suppressant agents. These polymers would also be useful as precursors for forming MoO3 films. Furthermore, in embodiments where lithium cations are utilized as the countercation, the compounds and polymers can be used to form new classes of materials for ion-conducting applications such as processable ion-conducting materials or polymer electrolytes. Finally, because the polyoxometalate monomers are relatively large and rigid, their use in forming polymers should restrict chain motions within the polymers thus enabling the polymers to be used to form materials which have high temperature uses.